Partial oxidation reactions, for example those required to produce a synthetic gas from natural gas, typically employ oxygen produced in a cryogenic plant for separating air by rectification and produce, for example, hundreds of tonnes of oxygen per day. Typically, the oxygen may contain up to 10% of impurities (mainly nitrogen). The air separation plant also produces nitrogen.
It is known to be advantageous to recover work from the nitrogen produced in the air separation plant. U.S. Pat. Nos. 2,520,862 and 3,731,495 disclose methods in which work is recovered from the nitrogen product, typically after compression, in a gas turbine. The nitrogen is employed to control the pressure in a combustion chamber associated with the gas turbine and thereby also reduce the rate of formation of oxides of nitrogen in the combustion chamber. The turbine may be employed to drive an alternator, and in this way electricity can be supplied to the air compressor employed in the air separation plant. Accordingly, most if not all of the energy requirements of the air separation process can be met. In UK Patent Specification 1 455 960 there is described an improved process for recovering work from the nitrogen product. This method involves a thermodynamic linking of the air separation plant with a steam generator. The nitrogen product is heat exchanged with flue gases intended for generation of steam in the steam generator so as to impart high grade heat to the nitrogen product and thus heat it to a temperature greater than 600.degree. C. The nitrogen product is then work expanded to convert most of its required heat energy into mechanical energy. Steam is generated by the flue gases downstream of their heat exchange with the nitrogen product. Residual, available heat in the work-expanded nitrogen product is used to preheat fluids entering the steam generator.
The process described in UK patent specification 1 455 960 has a number of drawbacks. First, the use of high grade heat to raise steam is relatively inefficient. Second, there is a significant additional capital cost involved in steam raising. Third, although there is the potential to use work recovered from the air separation process to generate large excess quantities of electricity for export, the process according to UK 1 455 960 does not avail itself of this possibility. Fourth, a suitable steam generation plant may frequently not be available on the site of the air separation plant.